Download Introduction to SQL, OleDB interface to Access from VB.NET

Introduction to SQL, OleDB
interface to Access from VB.NET
SQL
• Structured Query Language, abbreviated SQL
– Usually pronounced “sequel” but also “ess-cueell”)
– The common language of client/server database
management systems.
– Standardized – you can use a common set of SQL
statements with all SQL-compliant systems.
– Defined by E.F. Codd at IBM research in 1970.
– Based on relational algebra and predicate logic
SQL Data Retrieval
• Given an existing database, the SELECT
statement is the basic statement for data
retrieval.
– Both simple and complex, and it may be combined
with other functions for greater flexibility.
SELECT data_element1 [, {data_element2 | function(..)} ]
FROM
table_1, [, table_2, …]
[ WHERE condition_1 [, {not, or, and} condition_2] ]
[ GROUP BY
data_1, … ]
[ HAVING
aggregate function(…)… ]
[ORDER BY
data1, … ]
Or
*
SELECT statement
• Some sample aggregate functions:
– COUNT(*)
– AVG(item)
– MIN(item)
SUM(item)
MAX(item)
• Conditional Operators
–
–
–
–
–
–
=
<
>
<>,!=
<=
>=
Equal
Less than
Greater than
Not equal to
Less than or equal to
Greater than or equal to
SELECT Examples
• Select every row, column from the table:
– SELECT * FROM Orders;
– SELECT Orders.cust_id, Orders.prod_id, Orders.cost,
Orders.salesperson
FROM Orders;
• Returns a set of all rows that match the query
SELECT
• If a table has spaces or certain punctuation in
it, then Access needs to have the items
enclosed in square brackets []. The previous
query is identical to the following:
– SELECT [orders].[cust_id], orders.prod_id,
orders.cost, orders.[salesperson]
FROM Orders;
SELECT Query in Access
• Can flip back and forth between SQL View,
Run, and Design Mode
SQL
Run
Design
More SELECT Statements
• Note that we can have duplicates as a result of the selection. If we want
to remove duplicates, we can use the DISTINCT clause:
SELECT DISTINCT Orders.cust_id
FROM Orders;
• We can combine a selection and a projection by using the WHERE clause:
SELECT Orders.cust_id
FROM Orders
WHERE Salesperson = “Jones”;
• This could be used if we wanted to get all the customers that Jones has
sold to, in this case, CUST_ID=101 and CUST_ID=100. By default, Access is
not case-sensitive, so “jones” would also result in the same table.
More SELECT
• We can further refine the query by adding AND , OR, or NOT conditions. If
we want orders from Jones or from Smith then the query becomes:
SELECT Orders.cust_id
FROM Orders
WHERE Salesperson = “Jones” or Salesperson = “Smith”;
• Another refinement is to use the BETWEEN operator. If we want only
those orders between 10 and 100 then we could define this as:
SELECT Orders.cust_id, Orders.cost
FROM Orders
WHERE Orders.cost >10 and Orders.cost <100;
• Or use the between operator:
SELECT Orders.cust_id, Orders.cost
FROM Orders
WHERE Orders.cost BETWEEN 10 and 100;
More SELECT
•
Finally, we might want to sort the data on some field. We can use the ORDER BY
clause:
SELECT Orders.cust_id, Orders.cost
FROM Orders
WHERE Orders.cost >10 and Orders.cost <100
ORDER BY Orders.cost;
•
This sorts the data in ascending order of cost. An example is shown in the table:
CUST_ID
COST
102
15
100
20
101
30
•
If we wanted to sort them in descending order, use the DESC keyword:
SELECT Orders.cust_id, Orders.cost
FROM Orders
WHERE Orders.cost >10 and Orders.cost <100
ORDER BY Orders.cost DESC;
Joining Data from Multiple Tables
• If our data is in multiple tables we can join them
together in one query.
– Use a JOIN operator (Access default w/Design view)
– Add tables to the FROM, WHERE section (what we will use
here)
• Say we have the following table in addition to
Orders:
Multiple Tables
SELECT Orders.cust_id, Customer.Cust_Name
FROM Orders, Customer
WHERE Orders.cost >10 and Orders.cost <100;
Result:
100
101
102
100
101
102
100
101
102
Thomas Jefferson
Thomas Jefferson
Thomas Jefferson
Bill Clinton
Bill Clinton
Bill Clinton
George Bush
George Bush
George Bush
PRODUCT of two tables!
• What do you expect from this query?
Multiple Tables
• Need to link the tables by their common field,
the customer ID:
SELECT Orders.cust_id, Customer.Cust_Name
FROM Orders, Customer
WHERE Orders.cust_id = Customer.Cust_Id and
Orders.cost >10 and Orders.cost <100;
Result:
100
101
102
Thomas Jefferson
Bill Clinton
George Bush
INSERT command
• Allows you to insert single or multiple rows of
data into a table
• INSERT INTO table [(column-list)] [VALUES
(value-list) | sql-query]
INSERT examples
Given mytable(field1 as currency, field2 as text, field3 as integer):
INSERT INTO mytable (field1, field2, field3)
VALUES (12.10, “bah”,20);
Adds a new row to the table mytable
If you don’t specify every field then fields left out get the default:
INSERT INTO mytable (field1, field2) VALUES(24.2, “zot”);
Adds only for field1 and field2.
INSERT Examples
INSERT INTO ORDERS (CUST_ID, PROD_ID, COST, SALESPESON)
VALUES (103, ‘Y338’, 55, ‘Smith’);
INSERT INTO ORDERS (PROD_ID, COST, SALESPESON)
VALUES (‘Y638’, 155, ‘Smith’);
Second might be useful if the CUST_ID is an autonumber field
DELETE
• Delete will remove a row from the table.
• DELETE FROM table_name [WHERE searchcondition]
Examples:
DELETE FROM mytable1;
Removes all rows!
DELETE FROM mytable1 WHERE field1 > 100;
Removes only rows with field1>100
UPDATE
• Update lets you modify the contents of the data.
UPDATE table_name
SET field_name = expression [, field-name=expression …]
[WHERE search-condition]
UPDATE mytable SET field1 = 0.0;
Changes all field1’s to zero for every row!
UPDATE mytable SET field1 = 0.0, field2 = “woof”;
Sets field1 to 0 and field2 to woof for all rows!
If this is a violation, access will prevent it from happening
UPDATE mytable SET field1 = 25.0 WHERE field2=“foo”;
Only updates the field where field2 is “foo”
SQL Queries
• There are a lot more queries, but that should
give you an idea of what is possible and how it
is done
• Next we’ll go over an example that uses SQL
on an Access Database from VB.NET
– Uses OleDB which is different from the book
– Database access technology changes rapidly
OleDB in VB.NET
• Add to the top:
Imports System.Data.OleDb
• Set the connection string:
– This tells VB.NET where the database is and how to
connect to it:
Public Class Form1
Dim connectionString As String
Private Sub Form1_Load(. . .) Handles MyBase.Load
connectionString = "Provider=Microsoft.ACE.OLEDB.12.0;Data
Source=C:\Path\To\database.accdb"
End Sub
For Office 2007
Example Reading from the DB
Dim cn As New OleDbConnection(connectionString)
cn.Open()
Dim cmd As New OleDbCommand("SELECT * From Students WHERE Lastname >= 'M'", cn)
cmd.ExecuteNonQuery()
Dim reader As OleDbDataReader = cmd.ExecuteReader()
While (reader.Read())
Dim ID As Integer = Convert.ToInt32(reader("ID"))
Dim Name As String = Convert.ToString(reader("LastName"))
Dim DOB As Date = Convert.ToDateTime(reader("DOB"))
Console.WriteLine(ID.ToString() + " " + Name + " " + DOB.ToString())
End While
cn.Close()
Example Writing to the DB
Dim cn As New OleDbConnection(connectionString)
cn.Open()
Dim newLastName As String = "Washington“
' ID is auto-update field so its left out of the insert
' Put single quotes around String fields, # around dates
Dim sql As String = "INSERT INTO Students (LastName, FirstName, DOB, Address) " + _
"VALUES ('" + newLastName + "', 'George', #04/19/2005#, '999 C St.')“
Dim cmd As New OleDbCommand(sql, cn)
cmd.ExecuteNonQuery()
Console.WriteLine("Executed command " + sql)
cn.Close()